unreasonable jealousy?

i don't know why I feel like this but I feel super uncomfortable whenever my partner talks about the person that they went on dates with before we got together.

I'm comfortable when we talk about our exes, even the long term relationships we used to have. but somehow I can't stand to hear about this girl?

they stayed friends and I kinda feel a resentment towards this girl but I have no idea why?? she's a nice person and she's dating someone else now but I can't bring myself to like her??

what's wrong with me??